The invention relates to improvements in television cameras, and particularly to a circuit for automatically preventing raster rotation in image tubes due to adjustments of the electrical focus.
In the field of television cameras, vidicon type image tubes such as those used for pick-up tubes in television cameras have an inherent problem known in the art as "image rotation", caused by the adjustment of the electron beam focussing fields. The problem arises due to electron beam rotation within the tube; i.e., the fact that the beam proceeds from the cathode to the target of the tube in a spiral path, whereby changes in the electrical focus of the beam causes the scanned raster to rotate its position on the target. This in turn causes image rotation on the displayed picture. Moreover, the sensitivity of the rotation, i.e., the resulting image rotational error induced, is large compared to the degree of focus adjustment made. Thus, a totally unacceptable rotational error can result from an imperceptible change in electrical focus.
It follows that any change in focus control voltage, whether intentional or unintentional, greatly aggravates the problem of establishing camera spatial registration, particularly in a color camera. Therefore, if it is necessary to re-focus a tube in a color camera, it is almost imperative that a complete re-registration of the camera tube be made to correct for the image rotation on the target. Thus the interaction between focus setting and registration, etc., becomes rather complex.
The classic method of correcting image rotation due to electrical focussing is by direct mechanical rotation of the deflection yoke. This method is obviously cumbersome and time consuming, since the error is induced by adjustments made from a remotely located point, while the correction must be made locally at the camera head. The method is further complicated by the usual absence of adequate monitoring equipment at the camera head.
Single axis skew is generally utilized in color cameras to correct the lack of orthogonality which is generally inherent in most camera tube deflection systems. That is, one axis skew is used to correct the condition where the horizontal axis may not be at right angles to the vertical axis, or vice versa. The skew correction is utilized to make one axis orthogonal to the other axis prior to performing any registration, etc., corrections during tube operation.
Thus it follows that the more sophisticated camera systems such as those incorporating automatic control of registration, shading, etc., incorporate the individually adjustable electrical skew controls, which thus enable independent rotation of the vertical and horizontal scanning axes. Thus, after having focussed the tube, individual horizontal and vertical skew control adjustments may be made to correct the image rotation caused by the focussing procedure.
Although the latter system for correcting image rotation by means of independent axis skew adjustments is a significant improvement over the mechanical procedure for correcting same, it still generally requires two individual compensating adjustments to be made every time the electrical focus is changed.
The present invention simultaneously generates the required electrical rotational correction for both axes, directly from the electrical focus control voltage that induces the rotational error caused by adjusting the focus. The proportion of electrical rotational correction provided is selected to just cancel the raster rotational error induced by the focussing procedure. The net result is that the electrical rotational adjustment is accomplished automatically in response to the electrical focus control voltage to circumvent any resulting image rotation due to the focus adjustment.
To this end, vertical-rate and horizontal-rate sawtooth generators, whose output amplitudes and polarities are controllable by a single DC input control voltage, are coupled to an electrical focus control voltage source via suitable potentiometers at a common input. The amplitude and polarity sensitivities of the generators are selected such that when driven simultaneously by the single input control voltage which adjusts the electrical focus, their respective outputs are such that the horizontal and the vertical scan centering systems coupled thereto cancel the image rotational error induced by the electrical focus adjustment. Thus, electrical focussing may be performed without attendant image rotation.
Accordingly, it is an object of the present invention to automatically correct image rotation induced by a change of focus, in response to the electrical focus control voltage which induced the rotational error.
Another object is to generate the required electrical rotational correction simultaneously for both axes directly from the electrical focus control voltage.
A further object is to generate an image rotation correction which just cancels the rotational error induced by a change in the electrical focus control voltage, whereby an electrical focus adjustment is accomplished without the usual image rotation.